1. Field of the Disclosure
Embodiments disclosed herein relate generally to a centrifuge system for processing a fluid including solids and liquids. In another aspect, embodiments disclosed herein relate to a dual feed centrifuge system for removing solids from a fluid material. In another aspect, embodiments disclosed herein relate to a dual feed centrifuge system for removing solids from a drilling fluid material. In yet another aspect, embodiments disclosed herein relate to a method of separating solids from liquids in a fluid material using a dual feed centrifuge.
2. Background
Oilfield drilling fluid, often called “mud,” is typically a liquid having solids suspended therein. In general, the solids function to impart desired density and rheological properties to the drilling mud. The drilling mud can also contain undesired solids in form of drill cuttings from the downhole drilling operation that require separation.
Drilling muds may contain polymers, biopolymers, clays and organic colloids added to an oil-based or a water-based fluid to obtain the required viscosity and filtration properties. Heavy minerals, such as barite or calcium carbonate, may be added to increase density.
The drilling mud serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. Typically, the mud is mixed at the surface and pumped downhole at high pressure to the drill bit through a bore of the drillstring. Once the mud reaches the drill bit, it exits through various nozzles and ports where it lubricates and cools the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures, thereby preventing fluids from blowing out if pressurized deposits in the formation are breached. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) itself and the density (or its inverse, specific gravity) of the fluid used. Depending on the type and construction of the formation to be drilled, various weighting and lubrication agents, as mentioned above, are mixed into the drilling mud to obtain the right mixture. Typically, drilling mud weight is reported in “pounds,” short for pounds per gallon. Increasing the amount of weighting agent solute dissolved in the mud base will generally create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over-invade the formation. Thus, a drilling mud can be referred to as weighted or un-weighted, depending upon the amount of weighting agent and other additives contained therein.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit at the bottom of the borehole to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling mud exiting the nozzles at the bit acts to stir-up and carries the solid particles of rock and formation to the surface. Therefore, the drilling mud exiting the borehole from the annulus is a slurry containing formation cuttings.
Before the drilling mud can be recycled and re-pumped down through nozzles of the drill bit, certain solids, for example, the drill cuttings, must be removed. In general, the drilling solids can be separated from the drilling mud using various combinations of shale shakers, centrifuges and mud tanks.
One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker.” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges.
In some cases, the drilling mud fluid recovered from the shale shaker may be free from large drill cuttings and can be sent to a mud tank for further separation and processing. For example, the residual fluid can be further processed to form drilling mud for downhole reinjection. However, in other cases, the fluid effluent from the shale shaker may require further solids separation, such as to adjust the levels of or recover various additives from the drilling mud. Such further separation can be accomplished using a centrifuge.
The principle of the centrifuge operation relies upon the density difference between the solids and the liquids within the drilling mud. As a rotational torque is applied to a centrifuge generating a centrifugal force (hereinafter, “G force”), the higher-density solids preferentially accumulate on the outer periphery inside the centrifuge, whereas the lower-density liquids preferentially accumulate closer to the axis of the centrifuge rotation. Upon the initial separation by the G force, the solids and the liquids can be removed from opposite sides of the centrifuge using a ribbon-type screw conveyor, sometimes referred to as a scroll.
Referring to FIG. 1, a conventional centrifuge, such as that disclosed in U.S. Pat. App. Publ. No. 2006/0105896 A1, is illustrated. Centrifuge 10 has a bowl 12, supported for rotation about a longitudinal axis, wherein a large bowl section 12d has an open end 12b, and a conical section 12e has an open end 12a, with the open end 12a receiving a drive flange 14 which is connected to a drive shaft (not illustrated) for rotating the bowl 12. The drive flange 14 has a single longitudinal passage which receives a feed pipe 16 for introducing a drilling mud feed into the interior of the bowl 12. A screw conveyor 18 extends within the bowl 12 in a coaxial relationship thereto and is supported for rotation within the bowl 12. A hollow flanged shaft 19 is disposed in the end 12b of the bowl and receives a drive shaft 17 of an external planetary gear box for rotating the screw conveyor 18 in the same direction as the bowl 12 at a selected speed.
The wall of the screw conveyor 18 has a feed port 18a near the outlet end of the feed pipe 16 so that the centrifugal forces generated by the rotating bowl 12 move the drilling mud radially outward through the feed port 18a into the annular space between the screw conveyor 18 and the bowl 12. The annular space can be located anywhere along the large bowl section 12d or the conical section 12e of bowl 12. The fluid portion of the drilling mud is displaced toward the end 12b of the bowl 12 and recovered through one or more fluid discharge ports 19c. The entrained solids in the drilling mud slurry settle toward the inner surface of the bowl 12 due to the G forces generated, and are scraped and displaced by the screw conveyor 18 toward the end 12a of the bowl for discharge through a plurality of solids discharge ports 12c formed through the wall of the bowl 12 near its end 12a. The centrifuge 10 is enclosed in a housing or casing (not shown) in a conventional manner.
The main challenges facing the operation of a centrifuge include high feed rates and varying solids content in the feed. As the feed rates increase, high torque is typically required to accomplish the solids separation, thus resulting in increased costs due to equipment size, duplication, and increased energy costs. Additionally, feed inconsistencies due to variations in the solids content require constant torque adjustment, thus resulting in accelerated equipment wear and tear.
There is still a significant need in the art for improved centrifuge devices and methods for more cost-efficient solids separation from drilling muds that can handle high feed rates and varying solids content in the feed.